Prevent integer underflows in ID3::Iterator

If mFrameSize is less than or equal to getHeaderLength(), an integer underflow
will occur. This typically leads to a crash reading out of bounds in the
following code. Prevent this from happening by validating mFrameSize.

Also add NULL checks after references to ID3::Iterator::getData.

Bug: 23285887
Change-Id: I35eeda3c5349ebbd9ffb3ea49b79af6a940d1395